Adhell using Samsung Knox, DNS666 still need to run Android VPN even it's blocking ads in DNS level & run natively.
However, DNS666 provides better domain filtering sources & customization compared to Adhell.
Adhell | KNOX
pros :
1- free
2- no vpn annoying system notification.
cons :
1- ads blocking not strong enough
2- still showing white blank ads handler in some site.
3- not blocking Youtube ads
4- slow config
dns66 | VPN : now having problem with nougat & expect more flaws. see xda for more info.
Adguard | VPN (sori i need to bias on this

)
pros :
1- active dev support
2- strong ads detecttion & blocking
3- block in-video toutube ads
4- easy config & customization
cons :
1- not free
2- annoying dns notification (due to samsung config. sony/huawei dn hv this, only small key icon)
others are joke. for rooted, Adaway always the best.
Thanks for the explain and break down...for now i think i stick with adhell....